Rav said: “As soon as man goes forth from the study of Halacha to the study of Scripture he no longer has peace. And Shmuel said: “It means one who leaves Talmud for Mishnah.” And Rav Yochanan said: “Even from Talmud to Talmud.”
Rashi explains: “from Talmud to Talmud: from Talmud Yerushalmi to Talmud Bavli, since the former one is deeper.”